AI System Overview
March 2012
AI System Basics – Navigation
• Default Navigation System
– Triangulation
• 2D terrain-based navigation
– Navigation Modifiers
• Waypoints
• Flight
• Volume etc.
• Multi-layer Navigation System
• Smart Object System
– Allows AIs to move in a special way
AI System Basics – Decision Making
• Behavior Selection System
– Uses Behavior Selection Trees to select behaviors of AIs
• Goal System
– Executes atomic and composite goals of AIs
• Cover System
– Provides AIs with static and dynamic covers
• Smart Object System (again)
– Allows AIs to interact with their environment
• Interest System
– Allows AIs to perform intelligent actions when not alerted
AI System Basics – Tactical AI
• Tactical Point System
– Allows AIs to ask intelligent questions about their
environment (e.g. where to hide or attack)
• Faction System
– Determines which AIs are hostile to which other AIs
• Group Coordination System
– Uses Coordination Selection Trees to select group behaviors
• Formation System
– Allows AIs to move in formations
AI System Basics – World Interfacing
• Signals
– Trigger events and/or change behaviors
• Perception System – 2 options:
– Perception Handler (legacy, usually per-game)
– Target Track System uses configurable ADSR
envelopes to represent incoming stimuli
• Communication System
– Allows AIs to play sound/voice/animation events
Environment (1/4) - CryAction
• Game Object Model
– Entity, Movement Controller, Extensions
• Actor & Vehicle System
– Health, camera, IK, weapons, animation etc.
• Flow Graph
– Visual definition of game logic
• AI Debug Renderer
– HUD, geometric primitives, text labels, graphs etc.
Environment (2/4) - Editor
• AI Entities (AIs)
– Properties, Flow Graphs (FG Debugger available), scripts
• Entity Archetypes
– Templates for properties of individual AIs
• AI Shapes
– AI Territories, AI Paths, Forbidden Areas
• Navigation
– Navigation Modifiers for where Triangulation is not used
• Cover Surfaces
– CoverSurface anchors for where cover should be
• Visual AI Debugger
– Recording AI signals, active behaviors, goals, stimuli etc.
Environment (3/4) – Lua & XML
• Lua scripts
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Entity definitions (including entity Flow Graph nodes)
AI behavior definitions
Group behavior definitions
Library or shared Lua code (game rules, basic entities)
Blackboards to share information among groups or globally
• XML files
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Behavior/Coordination Selection Trees
Goalpipes
AI Communications
Items (e.g. weapons)
Environment (4/4) – Entity System
• Spatial queries: GetPhysicalEntitiesInBox
• AIs and vehicles are entities of the Entity System
• To spawn an entity, its Entity Class is required
– Defined either using .ent file in directory Game/Entities
– Or through a C++ call to RegisterFactory(…) in game code
• Entity Pool can be used to limit the number of active
AI actors (AIs) per each specified Entity Class
AI System Execution Context
• AI update is called every frame
• That said, AIs are “fully” updated only at ~10Hz
• Some AI subsystems use independent time-slicing:
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Pathfinding
Tactical Point System
Dynamic waypoints update
Smart Object System
Interest System
Dead bodies removal
• Some services can be called synchronously from
game code (e.g. Tactical Point System queries)
AI Objects Hierarchy
• CAIObject: Basic properties (entity ID, position,
direction, group ID, faction etc.)
• CAIActor: Basic perception & navigation, behavior
selection, coordination, blackboard, AI Territory
awareness, AI Proxy (of CryAction)
• CPipeUser: Goalpipes/goalops, smart objects,
pathfinding/pathfollowing/obstacle avoidance,
attention target, full-fledged perception
• CAIPlayer: AI System’s representation of the actual
(real) game player
• CPuppet: Aiming, firing, stances, covers –
full-fledged AI agent
• CAIVehicle: Vehicle-specific code
Navigation (1/3)
• Default Navigation
– Triangulation on terrain
• Uses cylindrical objects (e.g. trees) and Forbidden Areas
– Human Waypoints
• Waypoints need to be placed manually
• Connections can be generated automatically
– Flight Navigation
• Information about navigable volumes for flying entities
– Volume Navigation
• More general volume navigation, e.g. for ocean
Navigation (2/3)
• Multi-layer Navigation
Navigation (3/3)
• In general
– A* search is time-sliced to use 0.5 ms per AI frame
(configured in console variable “ai_PathfinderUpdateTime”)
– Pathfinding options: high priority, straight or partial path
– Human Waypoints update is a bit heavy but time-sliced
– Navigation graph is optimized but needs some memory
• Navigation generation
– Navigation data is generated offline, in Editor
– Multi-layer Navigation allows the regeneration of navigation
mesh as the designer modifies the map
Behavior Selection Tree – Overview
• Effectively a state machine
– Defines conditions to changes Behaviors (“states”)
• Behavior variables are used to choose behaviors
– They can be set in Lua or as a result of receiving a Signal
• Uses tree decision system with condition checking
– Conditions are specified as strings but can be more
complex than just checking a single variable
• E.g. "IsHurt and !InCover and HasTarget"
– If a condition fails traversal moves on to the next sibling
– If no leaf node is reached then no behavior switch is made
• The code can be shared among a number of BSTs
Behavior Selection Tree - Example
<SelectionTrees>
<SelectionTree name="FogOfWar" type="BehaviorSelectionTree">
<Variables>
<Variable name="IsFar"/>
<Variable name="IsClose"/>
<Variable name="AwareOfPlayer"/>
</Variables>
<SignalVariables>
<Signal name="OnEnemySeen" variable="AwareOfPlayer" value="true"/>
<Signal name="OnNoTarget" variable="AwareOfPlayer" value="false”/>
<Signal name="OnLostSightOfTarget" variable="AwareOfPlayer" value="false"/>
</SignalVariables>
<Priority name="Root">
<Leaf name="FogOfWarSeekST" condition="IsFar"/>
<Leaf name="FogOfWarEscapeST" condition="IsClose"/>
<Leaf name="FogOfWarAttackST" condition="AwareOfPlayer"/>
<Leaf name="FogOfWarIdleST"/>
</Priority>
</SelectionTree>
</SelectionTrees>
Behavior Selection Tree – Definition
• XML file which resides in Scripts/AI/SelectionTrees
• Use tag <SelectionTree>
• Multiple Behavior Selection Trees (BSTs) can be defined in the
same file, if really necessary
<SelectionTrees>
<SelectionTree name="FogOfWar" type="BehaviorSelectionTree">
[…]
</SelectionTree>
</SelectionTrees>
Behavior Selection Tree – Tags
• To define a BST, use tags <Leaf> and <Priority>
• Leaf name should be the same as that of the corresponding
behavior (or a Leaf Translation node)
• Leaves are evaluated (as defined in attribute “condition”) in
order. The behavior of the first leaf evaluated to true is
selected.
<Priority name="Root">
<Leaf name="FogOfWarSeekST" condition="IsFar"/>
<Leaf name="FogOfWarEscapeST" condition="IsClose"/>
<Leaf name="FogOfWarAttackST" condition="AwareOfPlayer"/>
<Leaf name="FogOfWarIdleST"/>
</Priority>
Behavior Selection Trees – Variables
• Variables can only be Boolean
<Variables>
<Variable name="IsFar"/>
<Variable name="IsClose"/>
<Variable name="AwareOfPlayer"/>
</Variables>
Behavior Selection Trees – Signals
• AI signals can be used to change the values of
specific BST variables
<SignalVariables>
<Signal name="OnEnemySeen" variable="AwareOfPlayer"
value="true"/>
<Signal name="OnNoTarget" variable="AwareOfPlayer"
value="false"/>
<Signal name="OnLostSightOfTarget" variable="AwareOfPlayer"
value="false"/>
</SignalVariables>
Behavior
• Can essentially be thought of as an FSM state
• Written in Lua and contains a function table where
each row defines the response to a Signal
• Scripts must be in “Scripts/AI/Behaviors” (or below)
• Each behavior script should contain a call to
function CreateAIBehavior(name, parent, def)
defined in AIBehavior.lua in order to be added to the
table of available behaviors.
Behavior – Example
Scripts/AI/Behaviors/NewAliens/AlienGrunt/AlienGruntAvoidImmediateThreat.lua:
local Behavior = CreateAIBehavior("AlienGruntAvoidImmediateThreat", "GruntBaseBehavior",
{
Alertness = 2,
Constructor = function(behavior, entity)
...
end,
Destructor = function(behavior, entity)
...
end,
AvoidThreat = function(behavior, entity)
...
end,
AvoidThreatCoverNotFound = function(behavior, entity)
...
end,
...
})
Goalpipes
• Goalpipe is a predefined sequence of atomic goals (goalops)
to be performed by an AI
• In Lua it’s easier to create goalpipes on the fly, if necessary
• In XML goalpipe definitions look a lot more intelligible
Example of an XML goalpipe:
<GoalPipe name="ApproachAndShootObject">
<Speed blocking="false" id="Run"/>
<FireCmd mode="Burst"/>
<LookAround blocking="false" lookAroundRange="360"
scanRange="5" interval="60" breakOnLiveTarget="true"/>
<Timeout intervalMin="1.0" intervalMax="4.0"/>
</GoalPipe>
Goalops – Some Examples
• Locate
• Locate an AI object/actor for further handling
• LookAt
• Look at a specified AI object/actor
• FireCmd
• Attack a specified AI object/actor in a specified manner
• Signal
• Send a signal to a specified AI actor/group/neighborhood
• Speed
• Stance
• Stick
• Approach/follow a specified AI object/actor
• TacticalPos
• Find a spot that match certain criteria (to hide, attack etc.)
Cover Surfaces
• Actual cover information is generated offline,
in Editor
Interest System
• If the player is able to secretly approach AIs, merely
playing idle animations may not suffice to look cool
• Interest System basically triggers predefined AI
Actions, as soon as idle AIs of applicable AI classes
find themselves near applicable (interesting) entities
• Examples:
• AI attentively examines a dead body
• AI attentively examines an abandoned car
• AI “finds” a good spot and uses his binoculars to
attentively examine the landscape
Tactical Point System (TPS)
Bill, on the right, finds a hidespot from a Woodsman
Tactical Point System – Overview
• Tactical Point System
– Allows AIs to ask intelligent questions about their
environment
– Is a structured query language over sets of points
in the world
– Is used for finding hidespots or attack points or
navigation waypoints
– Optimizes queries automatically and processes
them in a time-sliced manner
• Accessible through Lua, XML, C++
TPS Query Example
• Find a cover from referencePoint
AI.RegisterTacticalPointQuery( {
Name = "Find_Cover",
{
Generation =
{
hidespots_from_referencePoint_around_puppet = 20.0
},
Conditions =
{
reachable = true,
coverSuperior = true,
},
Weights = {
distance_from_referencePoint = -1.0,
distance_from_puppet = -1.0,
friendlyDistance = 1.0,
},
},
Faction System
Determines which AIs are hostile to which other AIs:
<Factions>
<Faction name="Players">
<Reaction faction="Grunts" reaction="hostile"/>
<Reaction faction="Civilians" reaction="friendly"/>
<Reaction faction="Assassins" reaction="hostile"/>
</Faction>
<Faction name="Civilians" default="neutral"/>
...
</Factions>
(Game/Scripts/AI/Factions.xml)
Group Behavior – Example (1/2)
<SelectionTrees>
<SelectionTree name="DefaultGroup" type="CoordinationSelectionTree">
<Variables>
<Variable name="AreaSearch" />
<Variable name="Sweep" />
<Variable name="InvestigateMemberDeath" />
</Variables>
<Priority name="Root">
<Leaf name="GroupAreaSearch" condition="AreaSearch" />
<Leaf name="GroupSweep" condition="Sweep" />
<Leaf name="GroupInvestigateMemberDeath"
condition="InvestigateMemberDeath" />
<Leaf name="DEFAULT" />
</Priority>
</SelectionTree>
</SelectionTrees>
Group Behavior – Example (2/2)
local Behavior = CreateAIGroupBehavior("GroupAreaSearch", "DEFAULT",
{
Constructor = function(self, group)
...
end,
Destructor = function(self, group)
...
end,
OnGroupTargetNone = function(self, group)
...
OnGroupTargetSound = function(self, group)
...
OnGroupTargetMemory = function(self, group)
...
OnGroupTargetVisual = function(self, group)
...
end,
...
})
AI Territories & Waves
• Control the number of active AIs at any time
through simple Flow Graph logic
• All AIs assigned to an AI Territory can be activated,
deactivated, and spawned using a single FG node
• Optionally, AI Waves are attached to AI Territories
and allow independent AI activations
• If AIs were marked to be Created Through Pool and
assigned to an AI Wave, the AI Wave handles Entity
Pool issues – e.g. loading/unloading – automatically
AI Debug Draw – (1/2)
• ai_DebugDraw
1: Basic info on AIs (selected by ai_DrawAgentStats)
74: All the Navigation Graph (can be slow)
79: Parts of the Navigation Graph around the player
• ai_statsTarget Goon35
– Detailed info for specified AI (e.g. Goon35)
• ai_DebugTargetTracksAgent Grunt1
– Perception information on AI Grunt1
• ai_Recorder_Auto
– Record AI activity in Editor game mode for AI Debugger
• ai_DebugTacticalPoints
– Debug TPS queries
AI Debug Draw – (2/2)
• ai_DrawPath Grunt1
– Draw the path of a specific AI (can be “all”, for all AIs)
• ai_DrawPathFollower
– Draw the actual path following
• ai_DrawSmartObjects
– Display Smart Objects, their classes and attributes
• ai_DebugDrawEnabledActors
– List currently enabled AIs
AI Debugger
• AI Debugger can track a number of AI features: active
behavior, active goalpipe, signal received, attention
target, comments created with AI.RecComment etc.
Scriptbind Functions
• Some AI functionality is available in Lua, for example
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AI.CreateGoalPipe
AI.Signal
AI.FindObjectOfType
AI.GetAttentionTargetType – Visual, Memory, Sound, None
AI.GetAttentionTargetAIType – Actor, Grenade, Car etc.
AI.GetRefPointPosition
AI.DistanceToGenericShape
AI.SetBehaviorVariable (change behavior)
AI.CanMelee
AI.RecComment (make comment for Visual AI Debugger)
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Questions?